This invention relates in general to stripline filters and more particularly to a means for stabilizing characteristics of a stripline structures against temperature variations.
Stripline filters are small in size and can be implemented at lower cost than alternative filter structures. A stripline filter is typically fabricated from two layers of a dielectric, having opposing inner and outer surfaces. Layers of conductive material cover each of the opposing outer surfaces and constitute ground planes for the stripline structure. The dielectric substrates enclose at least one resonator wherein one end is grounded and the opposite end is an open circuit. The length of the resonator determines the resonant frequency, and is derived from the following relationship: EQU I=c*f/(4.sqroot.Er.mu.r) (1)
where:
I=physical length of the quarter wave resonator; PA1 c=speed of light in a vacuum; PA1 Er=relative dielectric constant of substrate; PA1 .mu.r=relative permeability of substrate; PA1 f=lowest resonant fequency.
In addition to permeability and dielectric constant another parameter used to characterize a substrate material is the velocity factor Vf. Velocity factor may be readily derived from the following relationship: EQU Vf=1/(.sqroot.Er.mu.r) (2)
In dielectric (non-ferrite) materials, the relative permeability is unity therefore, Equation (2) reduces to EQU Vf=1/.sqroot.Er (3)
Thus velocity factor and dielectric constant of dielectric material follow an inverse relationship.
Accordingly, it can be concluded that in order to minimize the length of the resonator at a particular resonant frequency, materials having low velocity factors should be utilized. Ceramics such as Neodymium Titanate which have a relatively high dielectric constant (ER&gt;80) are currently being used in the construction of stripline resonators to allow fabrication of small stripline filters in applications such as pagers and portable two-way radios.
FIG. 1 illustrates a cross-sectional view of a conventional stripline structure 100 prior to completion of fabrication. The stripline structure 100 includes substrates 20 and 30 of an identical ceramic dielectric material having equal thicknesses. Substrate 20 includes opposed outer surface 20A and inner surface 20B, and substrate 30 includes opposed inner surface 30A and outer surface 30B. Ground plane layers 40 and 50 of electrically conductive material are situated on surfaces 20A and 30B, respectively, as shown. Two identical and substantially rectangular strips of conductive material 60 and 70 are disposed on surfaces 30A and 20B, respectively.
As shown in FIG. 2, conductive strips 60 and 70 are aligned and soldered together to form a resonator 80. One end of the resonator 80 is grounded, the opposite end is an open circuit (not shown), and the length of the resonator determines the resonant frequency of the stripline structure. Resonator 80 separates the dielectric substrates 20 and 30, thereby producing an air gap 110 within the stripline structure.
Use of ceramics with velocity factors in the range of 0.1 allow fabrication of stripline filters with favorable physical size in frequency ranges above 800 MHZ. However, to fabricate small stripline filters in the UHF (400-512 MHZ) or VHF (130-174 MHZ) frequency ranges, materials with lower velocity factors are needed. Unfortunately contemporary materials with low velocity factors exhibit excessive variation of velocity factor with respect to temperature and therefore are unsuited to construct a frequency stable, UHF stripline structure.